1. Field of the Invention
This invention relates to a heat exchanger having a superior durability against corrosion, and a fin material used in the same. More particularly, it relates to a heat exchanger useful when used for automobile purposes such as condensers and evaporators for car air conditioners, oil coolers, radiators and so forth, and a fin material used therefor.
2. Description of the Related Art
Aluminum alloys have been held important as materials for heat exchangers for use in automobiles and so forth, because they can achieve a high corrosion resistance by suitable treatment and can efficiently be joined by brazing which makes use of a brazing sheet. However, in recent years, for making automobiles have higher performance and adaptable to ecology, heat exchangers are required to be improved in performance so as to be more light-weight and have a higher durability, and techniques of aluminum alloy materials adaptable thereto are needed.
For example, in heat exchangers as typified by condensers and evaporators for car air conditioners, they are being made more light-weight by making their tubes, fins and so forth more thin-wall. Also, chromate type chemical surface treatment, which promises a high corrosion-proofing effect, shows a tendency of being removed because of environmental restrictions. Furthermore, there is an increase in factors that may accelerate the corrosion as exemplified by the use of snow-melting agents in a large quantity, the environmental pollution, the acid rain and so forth.
As a form of such heat exchangers for automobiles, what is used at present is a heat exchanger comprising combination of i) a fin material obtained by corrugating a brazing sheet cladded with a brazing filler material and ii) a tube produced by extrusion or the like; the both being joined by brazing. This tube is one intended for making a fluid such as a refrigerant flow therethrough, and hence may be fatal to its use as a heat exchanger if any leak comes about because of pitting corrosion. As a reliable means for keeping tubes from pitting corrosion, a method is commonly employed in which a Zn concentrated layer is formed on the surface of a tube by flame spraying or the like to make this Zn concentrated layer play as a role of a sacrifice material. Also, in order to make fins have some sacrificial effect, it is carried out to add Zn or the like to a fin material for the purpose of securing corrosion resistance of the tube.
Besides, the come-off or break-off of fins because of the corrosion of fins themselves or that at the joints between fins and tubes also come into question because it lowers the heat exchanging function. To the fins, external forces due to wind, washing water flows and so forth may also be applied during actual service. Accordingly, it is necessary for their strength to be retained so that the fins may not be broken even where the corrosion has proceeded to a certain extent. Especially where the fins are to be made thin-wall, it comes technically more difficult to secure the durability of the joints with fins themselves.
A method by which fins are prevented from coming off tubes is disclosed in Japanese Patent Application Laid-open No. 2004-170061 (Patent Document 1). This is a method in which, where the tube surface portion, tube core portion, fin and fin-tube joint of a heat exchanger are represented by A, B, C, and D, respectively, these are prescribed to have the relationship of A≦C≦D<B so as to prevent fillets from corroding predominantly, to prevent fins from coming off. As the tube, used is one made of an Al—Mn—Cu alloy with selected composition on the surface of which Zn has been flame-sprayed in a coverage of 2 to 8 g/m2. As a skin material for the fin, a brazing filler material is used which contains 0.1 to 0.3% by weight of Cu and 0.1 to 0.3% by weight of Mn. However, no particular technical consideration is taken in regard to the internal structure that shows grains and so forth of the core material of the fin.
The above Patent Document 1 has no disclosure at all as to the knowledge on any risk of the corrosion of fins themselves, in particular, grain boundary corrosion thereof that may adversely affect the function of heat exchangers like the come-off at joints, and as to any countermeasure therefor. Also, in industrial Zn flame spraying, the coverage of flame spraying may come non-uniform because of flame spraying portions and treatment chances. Hence, there is also a high possibility that areas come about in which the flame spraying is effected in a coverage as high as about 10 to 11 g/m2 which is beyond the 2 to 8 g/m2 that is the Zn flame-spraying coverage prescribed in Claims in Patent Document 1. Accordingly, this can not be said to be a technique which can achieve a stable product quality unless the product can be used without any problem even at the level of such a high flame-spraying coverage.
The present inventors have found that, where the Zn is flame-sprayed on tubes in a high coverage, not only the fin come-off tends to occur but also the grain boundary corrosion of fins themselves is accelerated. Accordingly, in order to tolerate any actual non-uniformity of the Zn flame-spraying coverage, it is necessary to make the fins not easily affected even when they become brittle because of the grain boundary corrosion.
Meanwhile, Japanese Patent Application Laid-open No. 2004-084060 (Patent Document 2) discloses a fin material (clad fin material) having superior brazing joinability to a tube material and a heat exchanger making use of this material. In regard to the grain boundary corrosion, it is stated that the grain boundary corrosion resistance of fins can be improved by controlling Si concentration at the surface and thickness center of a fin. How to specifically carry out brazing as disclosed in this publication is not necessarily definite. A method is disclosed in which the brazing treatment time is set within 15 minutes, and preferably within 10 minutes, during which the fin material is heated from 450° C. up to brazing temperature (about 600° C.) and then cooled to the solidifying temperature of solder. As disclosed in this publication, a fin is used in which its core material before brazing is of fibrous micro-structure and crystal grains of the core material after brazing are 50 to 250 μm in size, and it is stated that the feature that the core material has such micro-structure is a condition necessary for fins to be well joined to tubes.
As further disclosed in Patent Document 2, corrosion resistance and so forth are evaluated in Example by using test materials prepared by combination of pure-aluminum tubes surface-treated with Zn and clad fin materials. However, the surface Zn coverage that greatly influences the corrosion resistance after brazing is not specified. It is also disclosed that a brazing material for the clad fin material contains 0.1% by weight or less of Cu. However, any specific reason why the Cu is incorporated is unclear, and nothing is taken into consideration as to the corrosion resistance at brazing joints.